1. Field of the Invention
The present invention relates to a cooling system for cooling an electronic device such as a computer having components which generate heat during operation.
2. Description of the Related Art
Hitherto, various cooling systems have been proposed for cooling electronic devices. For instance, Japanese Patent Unexamined Publication No. 59-11654 discloses a cooling system which has a cooling air duct for cooling a semiconductor device, a by-pass duct extending in parallel with the cooling air duct, and a by-pass air supply port provided at a predetermined portion of the by-pass duct and opening to the cooling air duct.
Japanese Patent Unexamined Publication No. 63-84100 discloses a cooling system in which a partition wall is provided to form a by-passing flow of cooling air, the partition wall being provided at predetermined portions with apertures to concentate cooling air to devices which have specifically large heat generating rates. Japanese Utility Model Unexamined Publication No. 62-65895 discloses a system in which an air introduction or discharge mechanism is provided in a portion of a side wall of a cooling air passage so as to enable introduction of ambient air In a cooling system disclosed in Japanese Utility Model Unexamined Publication No. 62-192696, vent holes are formed in a plurality of printed boards which are arranged at a predetermined spacing. Japanese Patent Unexamined Publication No. 55-108800 discloses an arrangement in which an air flow regulator, i.e., a flow resistance member, having a plurality of apertures is placed at an intermediate portion of an air passage such that a part of the flowing air shunts from the main flow at the position where the regulator plate is located so as to form a by-pass flow which merges again with the main flow at a downstream position
Various types of heat-radiating fins also have been proposed. For instance, Japanese Utility Model Unexamined Publication No. 58-159795 discloses an arrangement in which each of a plurality of heat-radiating fins is provided with a slit which enables cooling air to flow perpendicularly with respect to the direction of the main flow of the cooling air. Japanese Utility Model Unexamined Publication No. 55-113353 discloses an arrangement in which a cylindrical stud is extended from a semiconductor device, and heat radiating fins are secured to the cylindrical stud perpendicularly thereto, with each heat radiating fin having a disk-like form of a diameter substantially the same as the length of the container of the semiconductor device and being cut along the longitudinal side of the container A cooling fluid is made to flow along the heat radiating fins and notches are formed in the fins along the flow of the cooling fluid In these known heat-radiating fin arrangement, no means has been provided for positively controlling the flow velocity distribution of the cooling fluid along the heat-radiation fins.
In a typical conventional cooling system for cooling a plurality of heat sources such as semiconductor devices arranged in series in a plurality of stages, a cooling fluid is made to flow unidirectionally from one to the other ends of the series arrangement of the heat sources In such a case, the cooling fluid temperature is gradually raised as the fluid is progressively heated so that the cooling effect is decreased towards the downstream end, resulting in a temperature distribution such that downstream heat sources exhibit higher temperatures than the upstream heat sources, as will be seen from FIG. 4. The known cooling systems mentioned previously are intended to divide the flow of a cooling fluid into a main flow component and a by-pass flow component and the by-pass flow component is merged into the main flow at a predetermined downstream portion of the cooling fluid path so as to control the temperature rising at the portion of the series arrangement of the heat sources downstream from the position where the by-pass flow component merges in the main flow component.
These known cooling systems provide an appreciable cooling effect in the region near the position where the by-pass flow component merges in the main flow component by virtue of the mixing effect caused by the mixing of flow components so that the temperature of the heat source in this region is effectively cooled to reduce its temperature as shown in FIG. 4. However, the aforementioned mixing effect terminates quickly and a single flow without mixing is produced so that no substantial improvement is achieved in the effect of cooling at the downstream side of the mixing region. Namely, the heat transfer efficiency is reduced to allow the temperatures of the downstream heat sources to rise, with the result that the performance of the heat sources such as semiconductor chips is impaired at the downstream side of the series of heat sources
Known heat-radiating fins, e.g., the fins disclosed in Japanese Utility Model Unexamined Publication No. 58-159795, have slits formed in the fins so as to allow cooling air to flow in the flow direction of the main flow component of the cooling air. The slits produce a so-called leading edge effect to improve the heat transfer efficiency. This type of heat-radiating fin, however, cannot provide any appreciable increase in the heat radiating rate because the heat radiating area is decreased due to the provision of the slit In order to make an efficient use of the heat radiating fins, it is necessary that the fin has a thickness which is large enough to avoid any drastic redution in the cooling efficiency. In addition, a greater fin height requires a greater plate thickness. The provision of a slit in such a thick fin undesirably increases the area where separation of the cooling fluid takes place at the leading end of the fin. This in turn causes the tremendous increase in the flow resistance, thereby failing to supply the cooling fluid at the desired level Referring now to the heat-radiating fins of the type disclosed in Japanese Patent Unexamined Publication No. 55-113353, mixing of flow components of the cooling fluid is less liable to occur because the cooling fluid flows between adjacent heat radiation fins. In consequence, the temperature difference between the cooling fluid and the heat sources is decreased towards the downstream end, thus failing to provide the required cooling effect on the downstream heat sources.
Heat radiation fins are usually made from metals having high levels of heat conductivity such as Cu, Al and so forth. The heat conductivity, however, has a definite value. Therefore, the heat radiation effect varies along the height of the fin. Namely, a large heat radiation efficiency is obtained at the base portion of the fin where the temperature difference between the fin and the cooling fluid is large but only a small heat radiation efficiency is obtainable at the free end portion of the fin where the above-mentioned temperature difference is small.